The overarching goal of this research is to obtain information about transmission of Grapevine leafroll-associated virus 3 (GLRaV-3), the primary virus species associated with spread of the economically damaging Grapevine Leafroll Disease (GLD) in Napa Valley. Such information is necessary to inform control strategies, it is clear that knowledge-based management of vector-borne diseases requires a robust understanding of how the pathogen spreads in vineyards. Mealybugs are the vectors associated with spread of GLD, but nothing is known about differences in transmission efficiency among mealybug species inhabiting vines in California. Furthermore, genetically distinct variants of GLRaV-3 exist but nothing is known about differences among these variants in terms of their ability to spread, or what the relevance of that variation is to GLD epidemiology. Lastly, all previous GLRaV-3 transmission studies were done under greenhouse conditions, and it is not known how well the results of such studies predict transmission in vineyards. This research addresses these significant gaps in knowledge.
We have completed proposed single and simultaneous mixed GLRaV-3 variant inoculations in greenhouse trials, using grape and vine mealybugs. Though two GLRaV-3 variants from singly infected source plants did not differ in transmission efficiency, the transmission efficiency of one variant was substantially lower when acquisition occurred from a co-infected source plant, indicating inhibition of transmission by the other variant. This may mean that one variant can be transmitted more efficiently than another and increase its incidence in the landscape (e.g. Napa Valley). It is not known whether some GLRaV-3 variants are more pathogenic than others.
We also set up an experiment in Napa Valley in summer 2011, inoculating 60 mature grapevines with GLRaV-3 using grape mealybugs as vectors. Each vine was inoculated using 10 first instar mealybugs, and then treated with insecticide two days later. Three months after inoculation, 20 of 60 plants tested positive for GLRaV-3 from our inoculations. No symptoms appeared in 2011. During the following growing season, GLD symptoms first began to appear in our experimental vines in June. By July, symptoms appeared in 29 of 60 experimental vines, and no other vines became symptomatic in 2012. Berry quality was affected in symptomatic vines compared to asymptomatic vines in the experiment.
Transmission in a parallel greenhouse experiment was higher than in the vineyard. In 2012 we set up a second field inoculation experiment in Napa to compare transmission of two different GLRaV-3 variants, by grape and vine mealybugs, in both Chardonnay and Pinot Noir. Inoculations were completed in July. Results from the 2012 inoculations are pending. This is the first time it has been shown that GLD symptoms due to mealybug inoculation of GLRaV-3 into established mature vines (~15 years old) in commercial vineyards are expressed in the following growing season. Results also showed that the entire vines were symptomatic in 2012, instead of just the inoculation site. Lastly, transmission success in the field was about 6%per individual mealybug.